The present invention relates to gas flow sensors employing a heated resistance wire, commonly called hot wire anemometers.
Numerous applications require measurement of the flow rate of a gas or mixture of gases. One such application is in medical apparatus, such as ventilators, for measuring the flow rate of inspiratory breathing gases supplied to a subject by the ventilator and the flow rate of the breathing gases expired by the subject. Measuring the flow rate of expired breathing gas is particularly difficult due to the wide range of instantaneous gas flow rates found during expiration, variations in the composition of the exhaled breathing gases, the moisture and sputum exhaled in the breathing gases, and for other reasons.
Hot wire gas flow sensors, or anemometers, have found use as expiratory breathing gas flow sensors in ventilators and similar equipment. In the simplest form of such a flow sensor, a thin, resistive wire, usually of platinum, is positioned in an airway flow conduit through which the expiratory gases pass. The wire typically extends transverse to the gas flow direction through the conduit. The platinum wire resistor forms one arm of a Wheatstone bridge circuit. The other arms of the bridge circuit contain other resistors, one or more of which may be variable. A power supply is connected across one pair of terminals of the bridge circuit and an indicator device is connected across the other pair of terminals of the bridge circuit.
Energization of the bridge circuit passes current through the platinum resistor to increase its temperature and cause it to become a xe2x80x9chot wire.xe2x80x9d The resistance of a platinum wire resistor, is proportional to its temperature. As the gas flows past the hot wire, the wire is cooled, altering the resistance of the resistor. The resulting resistance imbalance in the bridge circuit, as sensed in the indicator device, is an indication of the flow rate of the gas passing the hot wire resistor.
In another embodiment of such an anemometer, as the resistance of the wire resistor changes due to gas flow, the energization of the bridge is altered to keep the current through the wire resistor constant. The voltage drop across the resistor becomes an indication of the gas flow rate.
Or, as the resistance of the wire resistor changes responsive to gas flow, the current through the platinum wire resistor is adjusted to keep its temperature, and hence its resistance, constant. The resulting voltage drop across the resistor is the indication of the flow rate of the gas.
While highly suited as a means for measuring gas flow rates, certain problems have heretofore attended the use of hot wire anemometers. One problem is the inaccuracy of such sensors means at high flow rates. Because of the wide range of flow rates encountered in an application such as expiratory breathing gas measurement, this inaccuracy can be a serious problem. The inaccuracy is due in part to the fact that, for proper operation, the sensor requires an adequate amount of gain in the signal processing circuitry for the bridge circuit output signals. This gain drives the signal processing circuitry into saturation at the signal levels associated with high flow rates.
Also, as derived more fully below, the mass flow rate of a gas is related to the voltage across the wire resistor by the xc2xcth power. This means that the flow rate voltage signal curve is steep at low flow rates, but flat at high flow rates. This, in turn, causes the sensor to be highly accurate at low flow rates, but inaccurate at high flow rates. While the gain of the signal processing circuitry might be increased at high flow rates to improve high flow rate accuracy, this exacerbates the problem of saturation in the circuitry noted above.
A further problem that has been heretofore encountered with hot wire anemometers arises during replacement of the wire sensing resistor due to breakage or other reasons. The resistance of the wire sensing resistor depends on the length of wire forming the resistor. In practical embodiments of hot wire anemometers, the resistance of the sensing resistor inevitably varies from resistor to resistor. Replacement of a sensing resistor of one resistance with a sensing resistance of a different resistance has heretofore made it necessary to perform a calibration of the gas flow rate sensor, usually under field service conditions. To carry out the calibration most accurately requires a gas source that provides gas of a given composition over a known range of flow rates. This requirement has rendered calibration of hot wire gas flow rate sensors when the sensing resistor is replaced awkward, time consuming, and expensive.
A third problem is that the sensor characteristics are dependent on the composition of the gas or gases, the flow rate of which is being measured. When the measured gas is a mixture of gases, the mass concentration of the various components may vary. For example, when expired breathing gases are being measured, the composition may vary dependent on the oxygen uptake by the patient or the amount of carbon dioxide exhaled by the patient. To convert the mass flow measurement obtained by the hot wire anemometer to a volume flow measurement requires knowing the viscosity of the gas. While several techniques are available to obtain the conversion to volume flow, these tend to be rather complex and ill-suited for practical application.
It is, therefore, the object of the present invention to provide a hot wire anemometer and operating methods for same that overcome the shortcomings currently encountered with such devices.
It is a further object of the present invention to provide a hot wire anemometer flow sensor which exhibits a high level of gas flow rate measurement accuracy over a wide range of flow rates, including high gas flow rates.
A still a further feature of the present invention is to provide a technique for hot wire anemometers that avoids the need for a calibrating gas source when the sensing resistor is replaced, thereby greatly facilitating such replacement while, at the same time, maintaining the accuracy of the gas flow rate sensor.
It is yet another object of the present invention to provide a technique for hot wire anemometers that enables the anemometer to accurately determine gas flow rates for gases of differing composition.
The present invention obtains improved gas flow rate measurement accuracy for a hot wire anemometer of the constant temperature type over a wide range of flow rates, including high gas flow rates as follows. The signal processing circuitry for the anemometer contains a differential amplifier, a first input of which receives a signal corresponding to the voltage drop signal across the hot wire sensing resistor. A bias signal is provided to the second input of the differential amplifier to form a differential input signal to the amplifier. The magnitude of the voltage drop signal to the differential amplifier is sensed and the bias signal is established at a level that limits the magnitude of the differential input signal to a value not greater than one that which would drive the differential amplifier into saturation. Specifically, as the magnitude of the voltage drop signal increases, with an increasing gas flow rates, the voltage drop signal is compared with a plurality of reference values to provide a step-like increase in the magnitude of the bias signal. This ensures that the magnitude of the differential input signal to the amplifier is limited to a value that does not drive the differential amplifier into saturation, even at the high voltage drop signals associated with high gas flow rates. Saturation of the differential amplifier in the signal processing circuitry for the anemometer under these conditions is thus avoided. A signal is provided to output circuitry for the anemometer to provide appropriate scaling as the magnitude of the bias signal changes, so that the output of the anemometer is an accurate representation of the sensed gas flow rate.
The present invention also avoids the need to calibrate the sensor using a calibration gas source when a first sensing resistor is replaced with a second sensing resistor. To this end, the technique of the present invention employs the unique insight that the ratio of flow related components of the voltage drops across two different sensing resistors at a given gas flow rate is the same as the ratio of the voltage drops for the two resistors obtained at zero gas flow conditions. The magnitude of the latter components can be easily obtained from operation of the bridge circuit without the need for a calibrating gas source.
When the second sensing resistor is used to sense a gas flow rate, the ratio of the zero flow voltage drops for the first and second resistors is applied to the flow induced component of the voltage drop across the second sensing resistor resulting from gas flow past the resistor. This causes the flow component voltage drop for the second resistor to be the same as that which would have been obtained by the first sensing resistor. The operation of the gas flow sensor is thus not altered even though the sensing resistor has been changed.
The manner in which the foregoing is carried out is as follows. The bridge circuit containing the first sensing resistor is operated under zero gas flow conditions to establish a balanced condition in the bridge circuit at a desired current through the sensing resistor. The voltage drop across the first sensing resistor at such conditions is determined and retained as a compensating value. When the first sensing resistor is replaced with a second sensing resistor, the bridge circuit containing the second sensing resistor is operated at zero gas flow to establish a balanced condition in the bridge circuit with the desired current through said second sensing resistor. The second sensing resistor is exposed to the same gas, such as air, as the first sensing resistor. The voltage drop across the second sensing resistor under such conditions is determined. A ratio of the zero gas flow voltage drops so determined across the first and second sensing resistors is established which, when applied to the flow induced component of the voltage drop across the second sensing resistor when sensing gas flow, compensates the sensor for the replacement of the first sensing resistor by the second sensing resistor.
The present invention also provides a technique for compensating a gas flow rate sensor of the hot wire type for changes in the composition of gases, the flow rate of which is being measured. At zero gas flow, the bridge circuit containing the hot wire sensing resistor is operated with the sensing resistor exposed to gas of a first, known composition, such as air, and a balanced condition established in the bridge circuit at a desired current through the sensing resistor. The voltage drop across the sensing resistor at such conditions for gas of the first composition is determined. At zero gas flow conditions, the bridge circuit is operated with the sensing resistor exposed to a second gas for which the flow rate is to be measured. The voltage drop across the sensing resistor at such conditions for the second gas is also determined. A ratio of the voltage drops so determined across the sensing resistor is established which, when applied to the flow induced component of the voltage drop across the sensing resistor when sensing the flow of the second gas, compensates the sensor for changes in the composition between the first and second gases.
Various other features, objects, and advantages of the invention will be made apparent from the following detailed description and the drawings.